1. Field of the Invention
The present invention generally relates to improvements in dual mode cellular telephony and more particularly to a method and apparatus for repartitioning and compressing both digital and analog channels to maximize the capacity of the system without degrading the quality of the service.
2. Description of the Prior Art
The interim TIA standard IS-54 entitled "Cellular System Dual-Mode Mobile Station-Base Station Compatibility Standard" requires that the fixed component of the cellular telephone system be capable of handling mobile stations operating in an analog mode as well as mobile stations operating in a digital mode. The analog mode is at least similar to the Advanced Mobile Phone Service (AMPS) system which has been in operation in the United States for over 10 years. In the digital mode, Time Division Multiple Access (TDMA) is employed wherein, on a given carrier, time is segmented into frames and frames are further segmented into slots. A user may have exclusive use of one or more slots per frame. A voice channel is either two slots per frame (full rate coding) or one slot per frame (half rate coding). By this technique, several voice channels can occupy a given carrier. Since in the analog mode a carrier can only support one user at a time whereas in the digital mode a carrier can support three or six users at a time depending upon whether full rate or half-rate speech coding is used, the digital mode enjoys a traffic carrying advantage over the analog mode.
In order to comply with the IS-54 standard, a given base station will usually support carriers operating in any one of several modes. To more clearly describe the various communication modes, reference will hereinafter be made thereto as follows: a Type A communications mode designates a carrier operating in an analog mode, whereas Types B, C and D modes will designate carriers operating in digital TDMA modes, with Type B being a full rate coding of two slots per frame, Type C being a half rate coding of one slot per frame, and Type D being a higher capacity TDMA system using half rate speech compression, and digital speech interpolation (DSI). In the Type D mode, a channel is assigned only when the users are actually speaking. Such a system is more fully described in U.S. patent application Ser. No. 07/622,232 filed Dec. 6, 1990 and assigned to the assignee of the present application, the disclosure of which is hereby incorporated by reference fully herein. It should be understood that any combination of Types B, C or D communication modes may be used in this system without degrading the quality of service. A cellular base station needs access at any given time to some number of carriers to handle the traffic. It is desirable to maximize the system's capacity without degrading the quality of service. The factors impacting capacity and quality are the reuse factor, the propagation environment, the antennas, the distribution of mobile stations and the activity level of the channels. For a given reuse, propagation, antenna pattern and mobile station distribution, the Carrier to Interference Ratio (CIR) only depends on the activity level of the interfering carriers. Under these circumstances, if the parameters dictate failure to meet a CIR threshold, then the activity left with the carriers must be reduced until the performance goal is met.
The cellular model is idealized as a hexagonal grid with cell sites located at the center of each hexagon or at the vertices. Certain cells use the same frequency sets as other cells. This is called frequency reuse. In addition, cells may use directional antennas to limit interference, this is called sectorization. Popular configurations are seven cell, three sector and four cell, six sector. The capacity advantage achieved by frequency reuse is not without cost since it leads to co-channel interference. The CIR for a mobile station is found by computing the ratio of the desired signal power received by a mobile station to all of the co-channel interference received by that mobile station.
To the extent that a digital carrier supports a full complement of active voice channels, the interference generated by that digital carrier is justified. However, the standard requires that if any channel on the digital carrier is active, then the digital carrier must exist not only for the active channel but for the remaining complement of vacant channels on that digital carrier. Consequently, the interference generated by that digital carrier during the portion of the frame in which no voice or data traffic is being transmitted is, at least in some senses, unnecessary.
These particular factors raise at least two problems. A first problem is the appropriate mix of analog and digital carriers to be employed. While the IS-54 dual mode standard provides some flexibility in that a dual mode mobile station can operate either with an analog carrier or with a digital carrier, mobile stations that operate in only the analog mode should receive service to the extent that there are any available channels. If some a priori partitioning between analog and digital channels leaves some unused digital channels, then an analog mobile station may be denied service even though some channels are not being used, because that bandwidth has, a priori, been dedicated to the digital mode.
A second problem relates to the distribution of digital traffic on the digital carriers. Because the carrier can support multiple channels, and because the time at which a channel becomes vacant or inactive cannot be predicted, it is certainly conceivable that there will be multiple digital channels each operating at less than capacity. For example, if three digital carriers each operate at two-thirds of capacity, then there is the equivalent of a full digital carrier operating vacant. That condition generates unnecessary interference.